Resistor and method of making same



Aug. 19, 1941'. R. R. RIDGWAY RESISTOR AND METHQD 0F MAKING SAME FiledMay 24, 1939" 10 .100 Percentage of T010). Titania m 0 I EC 0 .P; m m m4e T n O Raymond Rfijdgfi (p e attorney wilnes s Huber! E-Covey PatentedAug. 19,1941

- UNITED STATES PATENT OFFICE I I 2,252,981 I v nasrsroa AND Mn'rnon orMAKING s Raymond B. moms, Niagara Falls, N. 12., ascignor to NortonCompany, Worcester, Mass., a

corporation of Massachusetts Applicatlon May 24,1939, Serial No. 275,366

BClaims.

The inventionrelates to resistor bars, compositions of matter for themanufacture of resistor bars and like products and methods ofmanufacture thereof.

n e object 'of-the invention is to provide a material .for themanufacture of resistor rods for electric furnaces. Another object ofthe inventionis to provide resistor rods of long life and goodelectrical properties. Another object of the invention is to provide acomposition for resistor rods for high power furnaces of desiredresistivity and great mechanical strength. Another object of theinvention is to provide a composition for the manufacture of resistorbars which will make bars of long life.

Another object of the invention is to modify aluminum oxide so that inthe fused condition it can be readily poured and on solidification willThe invention accordlngly'consists in the features of construction,combinations of elements, molecular and a crystalline structure,arrangements of parts, and in the several steps and arrangement andorder of each of said steps to one or more of the others thereof, aswill be ex' emplified in the product, method and apparatus to behereinafter described, and the scope of the application of which will beindicated in the following claims.

In the accompanying drawing:

Fig. 1 is a plan view of areslstor bar;

Fig. 2 isia sectional view of a resistor tube;

Fig. 3 is a resistance-temperature curve for a resistor embodying thisinvention; and t Fig. 4 is a curve showing the relation of specificresistance to the titania content of the resistor.

b conductive enough for use'as a resistor for electric furnaces andwill' still have great strength. Another object of the invention is toprovide a refractory conductor which will hold its electric conductiveproperties without deterioration. Another object of the invention is toprovide a resistor capable of resisting a substantial amount of thermalshock. Another object of the invention is to provide a dense homogeneousoxide resistor free from pores and blebs, with a surface free from weldsand discontinuities'. Another object of theinventlon is to provide aresistor having one or more of the characteristics indicated which willresist fusion up to a temperature of 1600 C. Another object of theinvention is to provide a bar which even at high temperatures may besupported. only which seals the interior thereof against further zAnother object of the'invention is oxidation. Another object of theinvention is to provide a tube which has a refractory non-conductivecoating integral therewith on the inside thereof so that it can act as arefractory container while the body of the tube acts as a resistor andradiates heat.

Another object of the invention is to produce a dense casting free fromvoids and gaseous inclusions. Another object of the invention is toprovide a method of manufacture of the products indicated which ispractical. Another object of I the invention isto provide a method ofproduction which will take care of the high shrinkages involved inrefractory oxide products of the nature dealt with herein. in partobvious or in part pointed out hereinafter.

Other objects will be In accordance with this invention, an electricalheating resistance element may be made of titania interfused and,crystallized with alumina. The titanium oxide of the resistor may be amixture of TiO: and T120: which may be in the form of the compositionand produced according to the method described and claimed in mycopending application Serial No. 275,365.filed of even date herewith. Asdescribed in said copendingv application, as the starting material forthe production of. the desired mixture of T102 and T120: I may meet andreduce purified titanium dioxide, T10: to form a crystalline mass ',ofT10: containing from 5 to of T120 and not over 5% of other refractoryoxides selected from the ture of my final products. Commercial ceramicgrade T10: ordinarily has thismuch S102 and I analyze samples toascertain that'my starting products contains such amount of silica.Preferably also my starting material has no more tha'n a trace of anyiron oxide, and the aforesaid commercial ceramic grade TiOs fills thisspeci-r fication. My starting material comes'in the form of a light,fluffy, amorphous'powder having a low apparent'density and iscomparatively free from moisture, having been calcined. I prefer a wellcalcined product. For the manufacture of partially reduced titaniumoxide referred to above as a mixture of T102 and T: I provide anysuitable open top electric arc furnace such,

for example, as.a Higgins type furnace as disclosed in United StatesPatent Reissue No. 13,027 to Aldus C. Higgins, but preferably I providea tilting furnace in order to be able to pour the molten charge. Thisfurnace preferably has graphite electrodes and any'suitable type ofregulating mechanism. Any known size of furnace is suitable for'carryingout the present inor bricks. I then crush these blocks or bricks tov agrain or powder. As the result of fusing the chemically precipitatedT102 in the open type electric arc furnace there is produced a mixtureof'titanium oxides probably consisting mainly of alumina will enable oneeasiest ture treatment '-,to remove the will. also do for the purpose.The foregoing are be considered as examples and any relatively pure tocarry out the present invention. e e s It is preferred to crush thealumina to a powder as well as the reduced titania. I feed the reducedtitania-and the alumina preferably in powdered or grain form eitherseparately or mixed to the Higgins. furnace at the rate of about onepound of oxides per eight-tenths kilo-- watt hour input. I usuallyoperate the furnace at about 65 to 75 volts with the electrodes ad-Justed to produce slightly reducing'conditions but not enough to reducethe oxides to metallic alloys. when the charge'is completed I hold thepower on until thebath becomes a complete TiO: and the remainderempirically identified as T1203. The fusion described reduces only partof the T102 to the suboxide represented by T120: and when carried out inthe manner indicated does not reduce any substantial part of thestarting material to the metallic form. With the rate of power inputindicated the product will correspond to an analysis of about 85% TiOzand 15% Ti-203. By using an open top electric arc furnace with graphiteelectrodes there is' produced an atmosphere of carbon monoxide on top ofthe melt formed by the burning of the electrodes,

and yet this is not pure nor is the atmospheric air so completelyexcluded that the reduction will go to an extreme extent. This fusion of.titanium oxides in the percentage as indicated when cooled is a fairlycoarse crystalline material having a dense black appearance and aconchoidal fracture and it is a conductor of elec- 'tricity, itsconductivity being along the order of that of poorly bonded carbon,carbonaceous materials, and the like.

This fusion of titanium oxides in the preferred form identified ashaving an analysis of about 85%. T10: and 15% T1203 is, so far as thepresent invention is more specifically concerned, the starting materialthereof. While substantially pure 'I'iOz has a resistivity of about11,730 ohms to the cm the partlyv reduced titanium oxide specified has aresistivity of about 340,000 micro ohms to the cm}. In other words thepure titanium dioxide 'TiO: has roughly thirty-five thousand times theresistance of the 85% TiOz 15% T1203 mixture.

I now charge the same Higgins furnace above referred to with the reducedtitanium oxide powder which is the starting material more spehomogeneousthin liquid under the arcs. I have I found in order to produce a densercasting with desired refractory fine grained structure that it isadvisable to shut ofi the power and allow the melt to stand in thefurnace crucible [for a few moments before pouring. This allows thetemperature to fall to a point where it is just above the crystallizingpoint of the liquid and when the melt is in this condition it is readyfor pouring and will'quickly crystallize to form thedesired castings,which may be shaped as shown in Figs. 1 and 2'or otherwise as desired.

In a typical run 100 pounds of the oxides were fused using power at therate of kilowatts.

The mixture was 11% titanium oxide and 89% aluminum oxide. The pouringtemperature selectedwas 1700 C. and the liquid was poured into a moldheated to 1000 C. I The resis orbars are dense and have a fine granularblue-black structure which is mechanically "strong andhas a linearcoeilicient of expansion of approximately 80x 10-".

v A microscopic study of the product shows that it consists of twophases, an intergrown phase of crystal alumina surrounded with acontinuous neti work of Ti0a.Th0a.Alz0a glass and slag which imparts theconductive properties to the mass. This reduced titania-rich phase seemsto difiuse into the alumina crystals imparting a blue coloration tothem. y

For electric furnaces whose temperatures range up to 1100" C. oxidationresistant-metal conducciflcally for this present invention and. somechemically pure prefused alumina preferably in the proportions of about12 parts of the reduced titania to 88 parts of the alumina. Suitablealumina (aluminum oxide, A1203) for use in carrying out this inventionmay be produced as described in my own prior Patent Reissue'No. 20,457reissued November 2, 1937.

I may also use alumina produced as described in Patents No. 1,798,261and No. 1,971,793 to Basil, T. Horsiield. Furthermore any alumina suchas produced by chemical precipitation and rather high temperalowered sothat it would bethe same as coppertors have usually constituted theheating elements. Above this'range, however, metal resistors have not ingeneral been very useful be-v cause they deteriorate rapidly with use.Above the range of 1100 C. silicon carbide resistor rods have beenchiefly used but such resistor-rods while more suitable than metalproducts nevertheless deteriorate'with use since they slowly oxidizeresulting in the gradual rising of theresistance. Resistor bars madeaccording to the present invention form in use a thin film of oxidizedmaterial, that is to say the T120: is probably re-oxidized to T102; Thisfilm which maybe of the order of one thirty-second of an inch thickprotects the remainder of the resistor bar and over a long period oftime no further oxidation takes place and the resistance value remainssubstantially constant.

It will be appreciated that in any resistor bar there is a desiredvrangeof resistivity. For example, one would not want the resistance tobe 7 under certain power conditions, not would one want the resistivityto be as high as that of aluminum oxide. have found that by addingaasaosr approximately 12% of the starting product for this'invention,which itself constitutes only about 15% T1201. a resistor bar isachieved which has the desired resistivity for a high power electricfurnace so that it can be embodied in bars whose cross-section is largeenough to give adequate strength and whose length is about right for allpractical considerations and furthermore the large proportion of aluminainsures a strong resistor rod which will resist mechanical stresses andstrains. At the same time a bar made as herein described is adequatelyresistant to thermal shock. In the resistor bar the conductive materialis the 'IJla-Os which is well dispersed as aforesaid.

Alumina is more.refractory than titania and the large percentage ofalumina in the resistor bars makes them. highly refractory and alsochemically stable. A typical resistor bar as described will resistsofteningor fusion up to 1600 C. and will absorb two kilowatts of energyand while supported only at its ends. The resistor rod or bar is avitreous fused oxide mixture and it has a clear metallic ring whenstruck with a hammer. Although as aforesaid it is resistant to heatshock this is a relative matter and being a I r the mold.

. v a 3 to avoid quick chilling of Using a graphite mold which has beenpreheated to 1000 C. or thereabouts I have formed dense, homogenous,well knit resistor bars. These may be shaped as shown in Fig. 1 toprorefractory container vide a central cylindrical Portion iii of adiamradiate it to the interior of an electric furnace vitreous orceramic object it should be heated and cooled slowly. Its modulus ofrupture at room temperatures has been determined to be 20,000 pounds asquare inch. It has a specific gravityof from 4 to 4.1. Its specificresistance is .3 ohm/in. at room temperatures.

The curve of Figure 3 shows the temperature coemcient of the resistance.The curve of Fig. 4 shows the changes in resistance of differentcompositions of alumina with varying percentages of titania, of which21% is T1203. For different specific uses the amount of the ingredientsA1203; T10: and T1203 will be varied. In order to produce resistor barswhich are strong, refractory, and have the desired electricalcharacteristics, the quantity of total titaniamay range between 5 and 20per cent, the balance being mainly alumina. Products having more than50% alumina are highly refractory and have a chemical stability whichmakes them useful for many industrial applications. The startingmaterial for this present invention and therefore also the finalresistorbar preferably contains -a small quantity of silica which causesthe mass to crystallize as a fine grained structure and results in asmoother and denser casting of improved conductivity. Zirconia acts alsoto improve the crystal structure.

With regard to the formation of the oxide film I have found that whenthe resistor bar is placed on power and raised. to a temperature betweenl200-and 1400 C. the oxidation of the partially reduced material forms aglaze which seals the interior of the structure against further oxidation. This glaze may be between one thirtysecond and one sixteenth of aninch thick and in a life test of a particular bar heated for tenthousand hours at 1100 C. the film at the end eter of ilve-eighths of aninch, cylindrical end portions ll of a diameter of flfteen-sixteenths ofan inch connected by frusto-conical portions II, the longitudinaldimensions being 13% inches overall length with the end portions 2inches and the central portion 6% inches, long. Also, as shown in Fig.2, the bar may be shaped as a hollow tube I 0 having face. I! and aninner cylindrical surface 20, or any other suitable shape may be made. Imay form tubes of the type'shown in -Fig.,2, and I have found that the I20 of the tubes and the resistor bar'- is hence a refractory container.For many heat treatment products it is a decided advantage to have aresistor rod which is also a refractory container since dissipation ofheat and Power can readily be reduced. For example I might pack theresistor tube within mflgnesia cement-and thus prevent it from radiatingvery much heat outwardly. As theheat would then radiate inwardly I couldheat treat small articles with a minimum amount of power consumption.

- Although the self-forming glaze, which has the refractory propertiesoffused alumina for example as sold under the trade-mark Alundum.

rent flows freely into and out of it. Although I have describedthe-production of rods and tubes in detail it should be understood thatI may also make other shapes for example plates which can used infurnaces as electric heating agents. Also although the articlesdescribed are electrical I resistors for power furnaces. neverthelessthe composition may be put to other uses.

Although silica has been described as the other.

ingredient besides two kinds of titanium oxide" and alumina, I mightsubstitute zirconia. silica is used the amount thereof. if the productis to be used as a resisto will be limited to a few per cent sinceotherwise the resistance is too reat.

I have found definite eflects from prefusing, casting and crushingthetitanium oxide as thereby a fine grained structure results. There is adistinct advantage, therefore, flowing from the method involving twofusing processes. However, so far as many features of the invention areconcerned, I may, still using two fusings. mix precipitated titania withalumina, fuse and reduce to form an ingot, then crush this ingot andrefuse toform the final melt which is poured into the mold. In thismodification of the process there are still two fusing operations of theoriginal ingredients. 1

It will thus be seen that there has been provided by this invention amethod and an article in which the various objects hereinabove set forthtogether with many. thoroughly practical advantages are successfullyachieved. As many possible embodiments may be made of the aboveinvention and as many changes might be made in the embodiment above setforthyit is to be understood that all matter hereinbefore set an outercylindrical .sur-

glaze forms on the insides,

' self-bonded crystalline 4- I 7 forth, or shown in the accompfl-livi sdrawing.

is to be interpreted as illustrative and not in a limiting sense, Iclaim: i I

. '1. An' electrical resistor'formed as a shaped.

self-bonded, coherent, dense, continuous, monolithic, prefused,crystalline structure having alumina crystals as the major phaseintergrown with a phase of crystals containing essentially.

titanium dioxide and a lower oxide and formin an electrically conductiverefractory body. 7

2. An electrical resistor formed of an intergrown crystalline structurecomprising. essenoxide, said ingredients being associated-as crystaisintergrown from a fused mass thereof.

- 5. An electrical resistor formedas a shaped, self-bonded, prefusedcrystalline body having alumina crystals as the major phase intergrownwith an electrically conductive phase of crystals tially a major phaseof crystalline alumina in- I terspersed with and surrounded by a lesseramount of an electrically conductive glassy phase containing T102, Th:and A120: crystallized-from a premised mixture of said oxides as adense, self-bonded, monolithic structure.

.8. .An electrical resistor formed as a shaped self-bonded crystallinestructure, 95% of which 1 is composed of crystalline alumina forming 80to 95% of the total mass. and titania. forming 5 to 20% of the mass. andwherein 50 to 95% of the titania is T: and the balance is a partiallyreduced titania empirically represented by the formula ThOa, saidingredients being associated as crystals intergrown from a fused massthereof. a

4. An electrical resistor formed as a shaped, structure containing from75 to 95% of-aluminsa. from 5 to 20% of titania and from zero to 5% ofingredients selected from the groupconsisting of silica and zirconia,the titania consisting of 50 to 95% of TiOrand the balance being a lowertitanium containing essentially titanium dioxide and a lower oxide, saidbody having a protective surface glare oi titanium dioxide thereon.

6. An electrical resistor formed of a crystalline mass, 95% of whichis'composed of alumina constituting so to 95% oi the total and titaniaconstituting 5 to 20% oi the total, the titanic. consisting of from toof titanium dioxide and the balance being a partially reduced titaniaempirically represented by the formula T1203. said resistor beinga'shaped dense body having a conchoidal fracture formed bycrystallization in a mold of a fused mixture of said ingredients '7. Themethod-of making an electrical resistor 7 portion thereof to a loweroxide, fusing said product with a larger amount of alumina andcrystallizing the interfusion as a shaped mass having an intergrowncrystalline structure.

RAYMOND R. RIDGWAY.

